Abstract: Both soil water content (SWC) and vapor pressure deficit (VPD) play an important role in regulating plant photosynthesis. Relative to the SWC, the impacts of VPD on gross primary productivity (GPP) are still largely unknown. In this study, we chose a water-limited temperate steppe in Inner Mongolia (NM station) and a humid but temperaturelimited alpine shrub-meadow in Tibet (HB station) to examine the responses of GPP to VPD. According to the ratio of SWC in a specific study period to multiyear average SWC value (RSWC), data in both sites were classified into three groups: driest 25% (the top 25%), middle 50% (the middle 50%), and wettest 25% (the bottom 25%). Our results showed that there were positive relationships between VPD and GPP under three SWC levels in HB station (P<0.05), without differences under the same SWC level (P>0.05). By contrast, GPP declined with increasing VPD under the driest and middle SWC levels in NM station, whereas such negative correlation was translated into a positive one under the wettest SWC level (P<0.05). Significant differences were found in the slopes of the relationships between GPP and VPD under different SWC levels in the NM station. When only considering the effects of VPD and SWC on GPP, SWC accounted for more than 60% of the variations in GPP under low SWC level in NM station, whereas VPD accounted for 90% of the variations of GPP in HB station. Our results revealed that the waterlimited grassland may be more vulnerable to high VPD in the future relative to the humid but temperature-limited ecosystems. Our results provide an important mechanism support and reference for how the vegetation productivity model couples the atmospheric drought effect into the model and also helpful for predicting the responses and adaptations of vegetation productivity to atmospheric drought in different ecosystems.

Key words: vapor pressure deficit, gross primary productivity, temperate steppe, alpine shrub-meadow, contrasting response.